1. Field of the Invention
The present invention relates to an improvement in an axial flow fan motor installed in a variety of OA (office automation) instruments or the like.
2. Description of the Related Art
In the equipment such as various OA instruments encasing a plurality of electronic circuits, there is a risk that the residual heat generated from the electronic parts may cause the damage of the electronic parts. Accordingly, a ventilation hole is provided in side walls of the casing, and the axial fan motor is mounted therein, thereby radiating the internal heat to the outside.
One example of an axial fan motor which has been conventionally used will now be described with reference to FIG. 9. Reference numeral 1 denotes a casing which is composed of a cylindrical venturi portion 2 having a slanted surface therein and a bearing box portion 3 formed integrally with the venturi portion 2. A pair of bearings 4 and 5 are mounted on the inner side of raised portions of the axial bearing box portion 3. The inner races of the bearings 4 and 5 support rotatably a shaft 6 made of metal. Reference numeral 7 denotes a retainer ring for preventing it from getting out of the shaft 6.
A knurling (D-cut) 8 is formed on an upper portion of the shaft 6 in FIG. 9. A central portion of a boss portion 10 of an impeller 9 is secured on the knurling. In order to secure the shaft 6 on the boss portion 10, the shaft 6 is inserted when the impeller 9 is molded. A plurality of fans 11 are radially mounted around an outer circumferential portion of the boss portion 10. The impeller 9 is inserted into the bearings 4 and 5 while being integrally formed with the shaft 6. Reference numeral 12 denotes a coil spring for providing a suitable thrust prepressure to the inner race of the bearing 5.
The boss portion 10 of the impeller 9 is shaped into a letter of E as shown in cross section in FIG. 9 and the fans 11 are mounted on the outer circumferential portion thereof. A yoke 13 is mounted on the inner side of the boss portion 10 for holding a cylindrical magnet 14 which has been magnetized in the circumferential direction. A disc-like printed circuit (PC) board 15 is mounted on the outer circumferential portion of the raised portion of the bearing box portion 3. As described later, the electronic circuits and parts are mounted as a DC brushless motor. Reference numeral 16 denotes a lead line for supplying the electronic circuits with electric power from the power source. A stator 20 is composed of a core 17, a coil 18 wound around the core 17, and an insulator 19 for holding the core 17, which is mounted at the position of the upper portion of the PC board and the inside of the magnet 14.
FIG. 10 is a diagram showing a drive circuit of the motor. Reference numeral 21 is a source terminal on a plus side. Reference numeral 22 denotes a source terminal on a minus side. The source terminal 22 is connected to a ground circuit. These source terminals 21 and 22 are connected to the lead line 16 (see FIG. 11). Cathode sides of diodes 23 and 24 are connected to the source terminal 21. An anode side of the diode 23 is connected to a terminal 25m of an integrated circuit (IC) 25.
Sixteen terminals 25a through 25p are provided in the integrated circuit 25. Although these terminals are the source terminals on the plus and minus sides and the input/output terminals for transmitting signals, parts of them are used as the drive circuit of the motor. The terminal 25o is connected to the ground circuit, and a capacitor 26 is connected between the terminal 25j and the ground circuit.
An anode of the diode 24 is connected to a source terminal 27a on the plus side of the hall device 27 for detecting a magnetic field of the rotor. A source terminal 27b on the minus side of the hall device 27 is connected to the ground circuit via a resistor 28. Output terminals 27c and 27d of the hall device 27 are connected to the terminals 25a and 25b of the integrated circuit 25, respectively. Coils 18 and 18 are connected between the terminals 25f and 25m and between the terminals 25h and 25m. Reference numerals 29, 30 and 31 denote connector terminals of the coils 18 and 18. The hall device 27 and the integrated circuit 25 are operative to control a current flowing through the coils 18 and 18 and to rotate the impeller 9 in a predetermined direction.
The thus constructed axial flow motor will operates as follows. Namely, when the power source voltage is fed to the source terminals 21 and 22 through the lead line 16, the current flows through the coils 18 and 18 by the action of the integrated circuit 25, s0 that the cores 17 of the stator 20 are excited in predetermined order. As a result, the impeller 9 is rotated around the shaft 6 due to the relationship between the magnetic field generated by the excited coil 17 and the magnetic field generated by the magnet 14. By this rotation, the fans 11 generates an air flow in a constant direction within the venturi portion 2 of the casing 1. Accordingly, it is possible to emit the heat to the outside, held within the casing encasing the axial flow fan motor.
When the current is caused to flow through the coils 18 and 18, a charging current corresponding to a value of that current will flow through the capacitor 26, so that the capacitor 26 is charged. When a rotation obstruction is occurred due to certain causes, the motor falls into a locked state, so that a large force is applied to the shaft 6, then, the charging voltage to be applied to the capacitor 26 is abruptly increased. When this voltage exceeds a certain level, the integrated circuit 25 cuts off the current flowing through the coils 18 and 18 to prevent the burnout of the coils 18 and 18.
FIG. 11 is a view showing an arrangement of parts of the electronic circuit shown in FIG. 10 on the PC board 15. As is apparent from FIG. 11, the PC board 15 is formed into a circular shape and is provided at its center with a circular hole 15a. The bearing box portion 3 penetrates the hole 15a. In FIG. 11, the same reference numerals are used to indicate the same components as shown in FIG. 10. These components attached to the PC board 15 are arranged to occupy an area approximately three fourths of the total area of the boss portion.
In the case where the above-described axial flow fan motor is applied to an OA equipment or the like, since the air is caused to flow along the shaft 6 by the impeller 9, by mounting the shaft 6 of the axial flow fan motor perpendicular to the wall of the casing of the OA equipment or the like, it is possible to effectively emit, to the outside, the air which has been heated within the casing. Therefore, there is an effect to restrict the temperature to be increased within a certain range. Namely, there is no problem in respect of the function of the fan motor, however, it has been required to curtail the cost and to further miniaturize the fan motor while keeping the same reliability and function as it is. Also, as one of the countermeasures for reducing the number of the mechanical parts, the capacitor 26, which functions when an excessive force is applied to the shaft 6 by some rotation hindrance, may be dispensed with, if this function is attained by another component.